There are two major causes--depletion of easily accessible oil in the West, and rising pressure on the West by the OPEC due to the United States' nation-building activities.
It's all a cause-and-effect kind of thing. As the United States angers other countries, those countries further limit their exports. This allows them to 'punish' the US and to make a good amount of money as oil prices surge. The only downside is that it further alienates them in the international community since they're hurting all of the non-OPEC economies of the world, but most OPEC countries don't care too much about that because they're already pretty isolated, politically.
